Mesoscale Precipitation Discussion 0116
NWS Weather Prediction Center College Park MD
739 PM EDT Fri Mar 29 2024

Areas affected...south-central CA coast into the Transverse Ranges

Concerning...Heavy rainfall...Flash flooding possible

Valid 292338Z - 300920Z

Summary...Heavy rain and possible flash flooding will affect
portions of the south-central CA coastline into the Transverse
Ranges this evening and tonight. Peak hourly rainfall of 0.7 to
~1.0 inches is expected along with 2 to 3 (perhaps a little
higher) inches of storm total rainfall through ~09Z.

Discussion...Visible satellite imagery from GOES West showed a
well-defined cloud pattern depicting a cyclone just off of the
north-central CA coast with an occluded/cold front nearing San
Francisco Bay at 23Z. Peak hourly rainfall in and around the Santa
Cruz Mountains over the past couple of hours has been in the 0.25
to 0.50 inch range. Water vapor imagery showed a closed mid to
upper-level low centered about 250 miles to the WNW of SFO with
strongly diffluent flow located east and southeast of the low
center. The closed low was tracking toward the southeast, helping
to keep the occluded/cold front moving along in a fairly
progressive fashion but short term heavy rain was still impacting
portions of the CA coast.

While the progressive nature of the occluded/cold front is
expected to continue through the night given no upstream impulses
observed on water vapor imagery to indicate stalling or slowing of
the front, locally intense short term rainfall rates are still
expected for portions of the CA coast into the Transverse Ranges
later tonight. Despite some wavering up/down, peak IVT values of
400-500 kg/m/s are likely to remain fairly steady through 09Z
along the coast and locations just inland but any instability
should remain low (<250 J/kg), lowering convective potential.
Nonetheless, strong forcing and low level winds becoming
orthogonal to the axis of the Transverse Ranges should favor
hourly rainfall totals over 0.5 in/hr at times within the axis of
highest IVT, with localized hourly totals near 1 inch possible.
Farther north, the duration of heavy rain is expected to be
shorter, limiting storm total rainfall into the 1 to 2+ inch
range. The Transverse Ranges will see the greatest potential for
rainfall totals in the 2 to 3 inch range through 09Z (although
spotty totals just over 3 inches may occur) along with the longest
duration of 0.5 in/hr rates, perhaps as much as 2-3 hours for any
given location within the terrain. The relatively quick addition
of 2-3 inches may result in excess runoff for a few locations from
the south-central CA coast into the Transverse Ranges with the
axis of heavy rain forecast to be located just west the Los
Angeles metro at 09Z.

Otto

ATTN...WFO...HNX...LOX...MTR...

ATTN...RFC...CNRFC...NWC...

LAT...LON   36382169 36312154 35872109 35592066 35482044
            35282015 35081977 34931948 34821925 34801891
            34801874 34721844 34361848 34011886 34221964
            34432071 35292145 36032186 36262184